Setting and origin for problematic rocks from the \3.7 Ga Isua Greenstone Belt, southern west Greenland: Earth’s oldest coarse clastic sediments
نویسنده
چکیده
Whether or not coarse detrital sedimentary rocks occur within the \3.7 Ga Isua Greenstone Belt (IGB), southern west Greenland, has been debated for some time. Repeated, regional metamorphic, deformational, and metasomatic events have obscured most protolith lithologies leading to misunderstandings about the stratigraphy and environments of deposition. Rocks here interpreted as meta-conglomerate crop out in a fault-bounded structural domain that is lower in strain relative to adjacent domains. The meta-conglomerate has a strike length of 1 km and is 10 m thick. Bed thickness ranges from 10 cm to more than 1 m, and beds may be either frameworkor matrix-supported. A poorly sorted and variably rounded polymict assemblage of framework clasts consisting of meta-chert, BIF, and a variety of mafic volcanic rock fragments are set into a matrix of biotite+quartz+garnet schist; clast compositions indicate reworking of adjacent stratigraphic units. A lack of structural similarity in framework clasts, range of grain sizes, range of rounding, and polymict composition demand a primary sedimentary origin for the deposit. Inferred depositional processes include traction and debris flow, which would be consistent with subaerial or shallow subaqueous environments, although the limited extent of the meta-conglomerate warrants interpretational caution. © 2000 Elsevier Science B.V. All rights reserved.
منابع مشابه
Protoliths of the 3.8–3.7 Ga Isua greenstone belt, West Greenland
The Isua greenstone belt (Fig. 1) contains the oldest known, relatively well preserved, metavolcanic and metasedimentary rocks on Earth. The rocks are all deformed and many were substantially altered by metasomatism, but both the deformation and metasomatism were heterogeneous. Transitional stages can be seen from relatively well preserved primary volcanic and sedimentary structures to schists ...
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